The present invention relates to methods and apparatuses for imaging by ionizing radiation, in particular in applications to medical imaging or to non-destructive inspection, and in particular using X-rays, or possibly xcex2 or xcex3 rays (radiography, tomography, scanning, etc.).
More particularly, the invention relates to a method of imaging by ionizing radiation comprising the following steps:
causing a detection signal S(t) to be generated by at least one detection cell (E1-En), said detection signal comprising a succession of individual detection signals s(t) generated by the detection cell each time said detection cell detects an ionizing ray coming from a certain observation zone and having energy lying in a predetermined range of energy values; and
processing the detection signal S(t) to obtain at least one measurement of the ionizing radiation flux detected by the detection cell during a measurement period T so as to generate an image of the observation zone, said image comprising at least one point given an intensity value which is a function of at least said ionizing radiation flux as measured by the detection cell during the detection period.
A method of that type is disclosed in document EP-A-0 845 687, for example.
It should be observed that the above-mentioned image, can:
be reduced to a point, in which case it constitutes a single measurement of the ionizing radiation flux; or
comprise a large number of points in two (or possibly three) dimensions, in which case it can give a visual representation of the zone under observation.
It should also be observed that the above-mentioned intensity value which in radiography or tomography generally gives an indication concerning the density of the tissue through which the ionizing radiation has passed, can either be a function of a single measurement of the ionizing radiation flux, or can be the result of combining a plurality of measurements (particularly when the method of the invention is applied to a scanner).
In known methods of the above type, the processing of the detection signal generally includes integration over a measurement time T, giving an integrated signal value which is proportional to the number of detections and thus to the ionizing radiation flux detected during the measurement period.
That method of measurement gives satisfaction at high levels of ionizing radiation flux. However, at low flux levels (e.g. fewer than 1 million counts per second), that method gives results that are less good because of the relative increase in noise level.
Furthermore, methods are also known of imaging by means of ionizing radiation in which the detection signal is not integrated, but in which individual detection signals generated by the detection cell are counted, thus making it possible to count directly the number of ionizing rays that have interacted with said detection cell. Such a method is disclosed, for example, by Babichev et al. (Digital radiographic installation for medical diagnostics, Institute of Nuclear Physics, Novosibirsk, 1989).
Such direct count imaging methods give good results at low levels of ionizing radiation flux, but poor results at high flux levels (e.g. greater than a few million counts per second), i.e. once the individual detection cell begins to detect ionizing rays so close together that it can no longer discriminate between two successive individual signals.
Thus, at present, there does not exist any method of imaging by ionizing radiation flux which gives results that are equally good both at low flux levels and at high flux levels of the ionizing rays.
An object of the present invention is thus to propose a method of imaging by ionizing radiation which makes it possible to obtain a reliable measurement of the ionizing radiation flux with a maximum signal/noise ratio over a very large range of flux levels.
To this end, according to the invention, in a method of the kind in question, during each measurement period, two measurements are performed simultaneously on the ionizing radiation flux detected by a given detection cell, specifically:
a first measurement f1 proportional to the integral of the detection signal S(t) generated by the detection cell during the measurement period T; and
a second measurement f2 proportional to the number of successive detection signals s(t) during the same measurement period T;
and the intensity value for each image point is a predetermined function of at least one pair of first and second measurements f1, f2 corresponding to said point, said predetermined function giving increasing weight to the first measurement f1 compared with the second measurement f2 with increasing value for a first estimate fe of the ionizing radiation flux, which first estimate is a function of at least one of the first and second measurements f1 and f2 (the predetermined function in question can be represented in particular by a mathematical formula or by a chart).
By means of these dispositions, a reliable measurement is obtained of ionizing radiation flux over a wide range of flux values (e.g. zero to 10 million counts per second). Furthermore, the reliability of this measurement, and in particular its high signal-to-noise ratio makes it possible to reduce the incident doses of ionizing radiation, which is of very great important for medical applications in particular. It can thus be expected that the doses delivered to patients for tomography can be divided by 2 or 3.
In preferred embodiments of the method of the invention, use can optionally also be made of one or more of the following dispositions:
the intensity value is a function solely of the ionizing radiation flux measurements corresponding to a given point of the observation zone;
the intensity value of said image point is a function of a flux value f of the ionizing radiation as determined by the following formula:
f=xcexxc2x7[xcex1xc2x7f1+(1xe2x88x92xcex1)xc2x7f2]
where xcex is a predetermined constant coefficient and xcex1 lies in the range 0 to 1, being an increasing function of said first estimate of the flux f (which function is optionally continuous, possibly includes horizontal portions, and need not actually reach the values 0 and 1 themselves);
xcex1 is a continuous function of fe;
xcex1 is a Boltzman sigmoid having the following formula:
xcex1=1xe2x88x921/[1+exp([fexe2x88x92f0]/xcex94f)]
where f0 and xcex94f are two predetermined values referred to respectively as the transition value and as the transition width;
fe is equal to f1 (at low flux levels, the difference between f and f1 is less than the difference between f and f2 at high flux levels, thus obtaining a relatively reliable first estimate of the flux f);
the image comprises a plurality of points each given an intensity value which is a function of a plurality of pairs of ionizing radiation flux measurements corresponding respectively to a plurality of adjacent points in the observation zone; and
the intensity value of each image point is a function:
firstly of ionizing radiation flux measurements corresponding to a given point of the observation zone; and
secondly of first and second gradient values corresponding respectively to the first and the second measurements in the vicinity of the said point of the observation zone.
Furthermore, the invention also provides apparatus for imaging by means of ionizing radiation, the apparatus comprising:
at least one detection cell adapted to interact with ionizing radiation having energy lying within a predetermined range of energy values to generate a detection signal S(t) comprising a succession of individual detection signals s(t), the detection cell being adapted to generate an individual detection signal s(t) each time said detection cell detects an ionizing ray coming from a certain observation zone and having energy lying in a predetermined range of energy values; and
processor means adapted to process the detection signal S(t) to obtain at least one ionizing radiation flux measurement detected by a given detection cell during a measurement period T so as to generate an image of the observation zone, said image comprising at least one point that is given a certain intensity value which is a function of at least said ionizing radiation flux as measured by the detection cell during the detection period,
wherein the processor means comprise measuring means for making two measurements simultaneously of the ionizing radiation flux during each measurement period, namely:
a first measurement f1 proportional to the integral of the detection signal S(t) generated by the detection cell during the measurement period T; and
a second measurement f2 proportional to the number of successive detection signals s(t) generated by the detection cell during the measurement period T,
and wherein said processor means further comprise computer means for determining the intensity value of each point of the image as a predetermined function of at least one pair of first and second measurements f1 and f2 corresponding to said point, said predetermined function giving increasing weight to the first measurement f1 compared with the second measurement f2 when a first estimate fe of the ionizing radiation flux increases, which first estimate is a function of at least one of the first and second measurements f1 and f2.